The present invention relates generally to the hot dip coating of a metal strip, such as a steel strip, with a coating metal such as zinc or aluminum, or alloys of each, and more particularly to a hot dip coating procedure which dispenses with the need for one or more strip guide rolls submerged below the surface of a bath of molten coating metal.
Steel strip is coated with a coating metal, such as zinc or aluminum, to improve the resistance of the steel strip to corrosion or oxidation. One procedure for coating steel strip is to dip the steel strip in a bath of molten coating metal. The conventional hot dip procedure is continuous and usually requires, as a preliminary processing step, pre-treating the steel strip before the strip is coated with a coating metal. Pre-treatment improves the adherence of the coating to the steel strip, and the pre-treating step can be either (a) a preliminary heating operation in a controlled atmosphere or (b) a fluxing operation in which the strip surface is conditioned with an inorganic flux.
When the steel strip has been subjected to preliminary heating in a controlled atmosphere, the strip may enter the hot dip coating bath at an elevated temperature which, in the case of a molten coating bath composed of zinc or zinc alloys, for example, can be at the same temperature as the bath of molten coating metal (e.g., 450.degree. C. (842.degree. F.)). When the pre-treating step is a fluxing operation, the steel strip can enter the bath of molten coating metal at a temperature ranging from ambient temperature up to about 450.degree. F. (232.degree. C.), for example.
Whatever the pre-treating step, the conventional hot dip coating procedure employs a coating step performed in a bath of molten coating metal containing one or more submerged guide rolls for changing the direction of the steel strip or otherwise guiding the strip as it undergoes the hot dip coating step. More particularly, the steel strip normally enters the bath of molten coating metal from above and moves in a direction having a substantially downward component, then passes around one or more submerged guide rolls that change the direction of the steel strip from substantially downward to substantially upward, following which the strip is withdrawn from the bath of molten coating metal as the strip moves in the upward direction.
A number of problems arise from the employment of guide rolls submerged in the bath of molten coating metal. These problems are described in detail in application No. Ser. 08/822,782 entitled "Hot Dip Coating Method And Apparatus" U.S. Pat. No. 5,827,576, and the description therein is incorporated herein by reference.
Certain attempts have been made to eliminate the employment of submerged guide rolls in a hot dip coating procedure. In these attempts, the steel strip is introduced into the molten coating metal through a strip passage opening in the vessel which contains the bath; the opening is located below the surface of the bath, and the strip is directed through the opening and through the bath along a straight-line path, which may be either substantially vertical or substantially horizontal. Conducting a strip through the bath along a straight-line path eliminates the need for submerged guide rolls to change the direction of the strip as it passes through the bath.
The strip passage opening is typically located in the bottom of the vessel containing the bath, or in a side wall of the vessel below the surface of the bath, and expedients are employed to prevent the molten metal in the bath from escaping through the strip passage opening.
Some expedients employ mechanical seals at the opening. These mechanical seals engage the side surfaces of the strip as it moves downstream through the opening, causing the seal to wear or break which in turn causes leakage of molten coating metal through the opening. Other problems associated with mechanical seals include large thermal gradients in the coating metal bath between the location of the seal and downstream locations, freezing of the bath, quality problems with the strip coating and irregularities in the coating thickness on the strip.
Other expedients employ electromagnetic devices that are located adjacent the strip passage opening and that develop electromagnetic forces which urge the molten metal in the bath away from the opening. When one employs electromagnetic devices to contain the molten metal at the opening, wear is not a problem (as it is with mechanical seals). Some electromagnetic devices prevent the escape, from the molten metal bath, of the overwhelming majority of the molten metal in the bath (bulk containment), but there is still some leakage or dripping of molten metal from the bath through the strip passage opening, particularly along the edges of the opening. In some cases, bulk containment may approach 98% or more, but in all cases, leakage is at the very least a significant annoyance if not a major problem.